Wednesday night, Harvey was a tropical depression with 35 mph winds. By Thursday midday, it was a Hurricane and Friday afternoon a category 4, major hurricane with 130 mph winds. By the time it made landfall, the storm was the strongest cyclone to strike the mainland United States in 12 years. What was behind this storm’s sudden increase in power?

The first is warm water. Warm water dumps heat energy into a developing storm. More heat allows the low-pressure circulating system to draw in more air from its surroundings, which makes the storm spin faster and grow. “A thick layer of warm water provides more fuel for the storm,” Camargo explains. The warm waters of the Gulf of Mexico, which Harvey is moving across, make it something of a breeding ground for hurricanes.

But warm water isn’t enough to cause a rapid intensification; it also depends on wind speeds throughout the troposphere (the lowest region of the atmosphere). If you think of the storm as a big tower, explains Camargo, you can’t have the winds blowing in one direction at the bottom of the storm but a different direction at the top—that would topple the column of air and the storm would be caput. The same thing happens if the winds have different strengths at different heights. Basically, having uniform winds from the bottom to the top of the storm allows it to grow in strength.

Harvey continues to dump historic rainfall on the Lone Star State, with flooding concerns to the north and east into Louisiana. The storm is back into the Gulf of Mexico, although little additional strengthening is expected.